The human
body readily responds to changing environmental stresses in a variety of biological and
cultural ways. We can acclimatize to a wide range of temperature and
humidity. When traveling to high altitudes, our bodies adjust so that our cells
still receive sufficient oxygen. We also are constantly responding in physiological
ways to internal and external stresses such as bacterial
and viral infections,
air and water pollution, dietary
imbalance, and overcrowding.

This ability
to rapidly adapt to varying environmental conditions has made it possible for
us to
survive in most regions of the world. We live successfully in humid tropical
forests, harsh deserts, arctic wastelands, and even densely populated cities with
considerable amounts of pollution. Most other animal and plant species are restricted to one or
relatively few environments by their more limited adaptability.

Humans
normally respond to environmental stresses in four ways:

biological
responses

1. genetic change

2. developmental adjustment

responses without
genetic change

3. acclimatization

4. cultural practices and technology

The first three
are biological responses. The last three occur during our lifetime
without further genetic change.

Genetic
Change

When an
environmental stress is constant and lasts for many generations, successful adaptation may
develop through biological evolution. Those individuals who inherit a trait that
offers an advantage in responding to particular stressesare more
likely to survive longer and pass on more of their genes to the next generation.
This is evolution through natural selection.For instance,
people whose ancestors have lived in areas that have had
endemicmalaria
for thousands of years often inherit some degree of immunity to this serious disease.
The high incidence of sickle-cell trait
among the people of Central Africa is largely the result of indirect selection for this
trait by malaria. Heterozygous carriers of the sickling gene usually do not have
sickle-cell anemia and are sufficiently resistant to the malarial
microorganism that they are at a selective advantage. Another example of
a genetic solution to an environmental stress is our ability to produce sweat
as an aid in cooling our bodies in hot environments. It is not surprising
that we have this capability because our
immediate prehuman ancestors were tropical
animals.

Genetic change in response to environmental stresses usually takes many generations to
become widespread in a population. Fortunately, we also have other ways of
responding more quickly as individuals during our own lifetime.
The word adjustmentsis used here to refer to these shorter term
physiological changes that are not inheritable. Theword
adaptationsis reserved for inheritable genetic changes developed in a population over a long period of time.

Developmental
Adjustment

One of the
more powerful types of adjustments to environmental stresses is a change in growth
patterns and development. This occurs in childhood and typically results in anatomical
and/or physiological changes that are
mostly irreversible in adulthood. Such permanent changes are referred
to as developmental adjustment or developmental acclimatization.

X-ray of an early
20th centuryChinese woman'sbound foot
The growth was stunted and
the bones were significantlydeformed so that they could
fit into a tiny pointed slipper.

Among humans,
developmental adjustments result from both natural environmental pressures and cultural
practices. An example of the latter was the now illegal custom
in China of tightly wrapping or binding the feet of young
girls with cloth in order to
hinder normal growth. While this caused permanent, crippling deformities of the
foot bones, it also resulted in extremely tiny feet which were considered to be very
attractive. Parents crippled their daughters with good intentions.
Small feet would make them more attractive marriage partners for rich important men
and save them from a life of drudgery.

Late 19th century
British
actress
(Lillie Langtry)
who typified the
ideal of
beauty
with her wasp-
shaped waist
achieved
with a
tight corset

It is easy to condemn the
old Chinese custom of foot binding as being barbaric. However, it is worth
considering that North Americans and Europeans have
intentionally altered parts of the bodies of their children and themselves with unpleasant
procedures as well. In the late 19th century, tight corsets worn by
girls when their bodies were still growing had the effect of deforming lower
rib bones dangerously in towards their lungs. Some rich women even had
lower ribs surgically removed in order to achieve a stylish "wasp-shaped"
waist. A 19 inch circumference was the ideal.

Intentional deformation of parts of the body is not something that only happened in the past.
In China today, there is a growing concern among many upwardly mobile middle
class men and women that they are too short. Thousands have sought a
solution to this problem by having their legs lengthened. This is
achieved by a long, painful process involving the surgical breaking of the
two lower leg bones in both legs and then using adjustable metal braces that
are anchored with steel pins implanted in the bone just below the knees and
above the ankles to progressively extend the length by nearly 1/16 inch
(about 1 mm.) a day as the bones heal. This widens the gap at the
break areas, thereby stimulating new bone growth. As a result, the
average patient permanently adds about 3 inches (7-8 cm.) to their height in
half a year.

Intentional body deformation is a common practice in North America today as
well. It is customary for middle and upper class parents to have the
teeth of their children straightened with retainers and braces. This is a long,
costly, and somewhat painful experience that alters the alignment of teeth. In part, it is done to
preserve and improve their functioning. However, a strong
motivation is to enhance appearance. Nose straightening and other
forms of plastic surgery are often done for the same reason, despite the
fact that they are painful. It is assumed by parents that these kinds
of body
alteration will increase the likelihood that their children will grow up to be more successful in life. This was
also the motivation of rich Chinese parents in the past who bound the feet of their
daughters and of contemporary Chinese who undergo leg lengthening.

Wearing fashionable
high-heeled shoes
can cause foot
deformities and other
orthopedic problems
over time

Permanent
changes to the shape of body parts may be unintended. For instance, wearing leather
shoes that enclose the feet makes them narrower than they would be otherwise. Similarly, the practice of women wearing shoes with pointed toes, high heels, and
often too small of a size commonly result in a number of painful orthopedic
deformities. Driving this outwardly illogical Western cultural practice is the
belief that small feet are attractive for women. The
American Academy of Orthopaedic Surgeons has reported that 9 out of 10 women
in the United States wear shoes that are too small for their feet, and 7 out
of ten subsequently have developed painful bunions, hammertoes, or other foot
deformities.

What makes
such developmental adjustments to parts of our bodies possible is the fact that humans
have a high degree of physiological plasticity. That is to say, we can be
physically molded by our environment during the growing process. Adults are the
result of genetically inherited traits that were shaped to a certain degree in each of us
by our environment as we grew up.

Child with marasmus

Extreme undernourishment or
overnourishment
in early childhood can result in devastating developmental adjustments.
When there is a prolonged shortage of food, as is experienced in a famine
situation, people can develop marasmus
(from Greek meaning "to waste away"). Symptoms include extreme
emaciation, diarrhea, anemia, and apathy. Women with marasmus usually
stop ovulating and, subsequently, cannot become pregnant. The loss of insulating body fat makes people with marasmus highly vulnerable to death resulting from a drop in
core body
temperature when the air falls below 60-65º F.
(15-18º C.). Young
children who survive marasmus usually end up with
short adult stature and some degree of mental retardation.
Unfortunately, marasmus is not a rare occurrence around the globe today.
Approximately 31 million children die each year from undernutrition, and 178
million are stunted in their growth. About 1 billion people are now
undernourished, and essentially the same number are overnourished and suffer
the health problems related to obesity.

Child with kwashiorkor

A lack of specific kinds of nutrients can
result in other life threatening health problems. For instance, when
babies and very young children have a diet that is extremely low in protein, they will likely
develop kwashiorkor. Also
contributing to this condition is an inadequate consumption of vitamins A and E as well
as the minerals zinc and selenium. Typical symptoms
of kwashiorkor include edema (or
swelling) due to fluid retention (especially in the abdomen), stick-like legs
and arms with little fat or muscle mass, apathy, and loss of hair and skin
pigmentation in patches. As in the case of marasmus, children with
kwashiorkor are likely to be apathetic and have impaired
immune systems which reduces their ability to fight off infections. If
a child survives kwashiorkor, they are likely to have their growth retarded.

Even specific
vitamin deficiencies alone can result in serious health problems
for children despite otherwise adequate diets. For example, a lack of
vitamin D can cause the bone disease known as rickets,
while inadequate amounts of vitamin A can
cause permanent blindness and
impair the immune system. Nearly 100 million
people in the world today have vitamin A shortages. Most live in Asia.
In order to reduce this deficiency, a new strain of genetically modified rice
("golden rice") that has relatively high amounts of vitamin A is now grown
extensively in Asia. However, a diet that has too much vitamin A is
equally harmful. It can cause birth defects (especially cleft palate)
and can interfere with the cells that produce new bone, resulting in a
dramatic increase in the risk of fractures.

Contemporarytall Japanese

Developmental
adjustment does not only result in defects and disorders. Dietary changes also can have
a positive effect if nutrition is
improved. This has been the case in Japan since the end of World War
II. The Japanese Education Ministry reported that
children have been significantly taller in each generation since then. In
1986, for instance, 14-15 year old Japanese boys averaged 7 inches taller than did
comparable aged boys in 1959. A key changing factor
in Japanese lifestyle has been diet. It is likely that this was
mostly responsible for the increased body size. Between 1961
and 1971,Japanese consumption of animal protein
rose 37% while plant food consumption dropped 3%. In the cities of Japan
and other increasingly affluent areas of East Asia, food habits have changed
dramatically over the lastseveral decades.
Hamburgers, pizza, fried chicken, and other high fat Western foods are very popularwith the young and affluent. In Japan today about
one fourth of the calories consumed are fat--this is 5 times higher
than just after WWII. Lending support to the hypothesis
that diet changes of this sort can result in significant developmental
adjustments is a recent
two year study of children in Kenya. It found that the inclusion of
only 60 grams (about two spoonfuls) of meat a day to the diet of young
children resulted in the development of 80% greater upper-arm muscle
compared to children who were strict vegetarians. A diet that included
a comparable amount of milk instead of meat resulted in an increase of 40%.
Foods of animal origin are important in the diet of young children because
they contain nutrients that are difficult to get from non-meat or non-dairy
sources. However, too much animal protein and fat can result in
obesity and other health risks.

Acclimatization

All other
forms of adjustment to environmental stresses are usually reversible whether
they occur in
childhood or adulthood. These reversible changes are
referred to as acclimatization or acclimatory adjustment. It is useful to consider
the different forms of acclimatization in terms of the length of time over
which they can occur.

Forms of
Acclimatization

Tanning is acommon form
of seasonalacclimatization

An
example of a long-term acclimatization is
people who lose excess body fat and are very slender
as a result of mild, long-term undernourishment.
If they later increase their diet to a consistent level
of excessive calories, they will very likely retain more body fat and eventually become
obese. They experience long-term acclimatization when they initially lose body fat
and again later when they retain it. In both cases, they are acclimatizing to the
available food supply.

Anatomical
and/or physiological adjustments also may develop over even shorter time periods.
For example, many people acquire dark skin tans during the summer months and lose them
during the winter. This change in skin coloration is a seasonal acclimatization
to the destructive effects of ultraviolet radiation from the sun.

When skin
divers descend into the ocean, they experience rapidly increased water pressure.
Within seconds, they can suffer from excruciating pain in their ears due to the unequal
pressure inside and outside of their eardrums. They must equalize this pressure by
blowing hard through their nose. By doing this, they are making a short-term
acclimatization to the changed environment.

When
traveling to a high altitude, it is common to experience a progressive drop in
the ability to hear due to a pressure inequality from one side of the
ear drum to the other causing it to slightly balloon out
and become less flexible. This difference in pressure
experienced in mountains can usually be
cancelled out by yawning, swallowing, or chewing gum. However, if
someone has a stuffy nose, it is often difficult to equalize the pressure
when they ascend to high altitude.
As a result, they are liable to have considerable pain in their ears.
This was a major problem for flyers prior to the time when airplanes were
pressurized.

The difference between the kinds of acclimatization is
not only in the
amount of time it takes for the adjustment to initially occur.
Usually, the shorter the time
for acclimatization, the quicker it is also in reversing once the environmental
stress is no longer present.

Combined Effects

Genetic
adaptation and the three types of adjustments to environmental stresses are not always
distinct phenomena. Acclimatization occurring in childhood may result in permanent
anatomical changes, as is often the case with malnutrition. When an acclimatization
is successful in providing good health and longevity, it can give individuals a selective
advantage in passing on their genes to the next generation. This can have a strong
determinant effect on the direction of evolution. In turn, genetic change can play a
significant role in adjustment since the ability to acclimatize is ultimately dependent on
genetic makeup.

Adaptability
to specific environmental stresses varies from person to person and from population to
population. We are not all biologically equal. For instance, some groups of
people are more successful in adjusting to high altitudes. Others can better handle
intense heat and high humidity. Adaptive responses tend to occur in
spatial clusters around the world. Usually, the most efficient adaptations for
specific environmental stresses are found in areas where those stresses are most
common. This is evidence that natural selection has occurred in the successfully
adapting population.

Cultural
Practices and Technology

It is
important to remember that humans do not only interact with their environments
biologically. We use culture as well. Over the last half million years
at least, we
invented technological aids that allowed us to occupy new environments without having to
first evolve biological adaptations to them. Houses, clothing, and fire permitted us to live
in temperate and, ultimately, arctic regions despite the fact we still essentially have the
bodies of tropical animals.

FA-18 fighter pilot
using
culturaltechnology to adapt
to high altitude

This does not mean, however, that human-made technology eliminates the biological
adaptive advantages of particular individuals or groups. People who have thicker
layers of fat insulation under their skin still usually survive better in cold climates,
while people who are slender do better in hot ones.

In the next four sections of the tutorial
you will learn how our bodies respond to several common kinds of environmental stresses.